Method for preparing a silica suspension in a vulcanisable silicon matrix to form elastomers

ABSTRACT

The invention concerns the preparation of a silica suspension in a silicon fluid, said suspension being used to produce silicon vulcanizable by polyaddition (RTV elastomers). The problem which the invention aims to solve is that of finding a technical compromise between the rheological and mechanical properties of the final RTV. The invention solves the problem by providing a method for preparing a silica suspension treated with hexamethyldisilazane (HMDZ) in a silicon fluid with siloxyl Si-Vinyl function, characterised in that it consists essentially in introducing HMDZ in the preparation medium, before and/or substantially while bringing at least part of the silicon fluid prepared with at least part of the particulate filler used, said introduction being carried out once or several times for a HMDZ fraction corresponding to a proportion not more than 8% by dry weight with respect to the total silica charge and after bringing together the POS and the filler.

This application was a national stage filing under 35 U.S.C. §371 ofInternational Application No. PCT/FR98/01319 filed on Jun. 23, 1998,which International Application was published by the InternationalBureau in English on Dec. 30, 1998.

TECHNICAL FIELD

The field of the invention is that of silicone elastomers which can beobtained by polyaddition and the main components of which are siliconepolymers and fillers.

More specifically, the present invention relates to the preparation ofan intermediate product useful for obtaining these silicone elastomersand consisting of a suspension of a reinforcing filler in apolyorganosiloxane carrying Si-alkenyl—preferably Si-Vi—functionalgroups capable of reacting by polyaddition with the SiH crosslinkingfunctional groups of another POS.

PRIOR ART

A distinction may be made between reinforcing and non-reinforcingfillers in silicone rubbers.

The most widely used reinforcing fillers are preferably pyrogenicsilicas having a BET surface area >50 m²/g. They owe their reinforcingeffect firstly to their morphology and secondly to the hydrogen bondswhich form between the silanol groups on the surface of the silicas(3-4.5 SiOH groups/mm 2) and the polyorganosiloxane (POS) chains. Theseinteractions between the filler and the polymer increase the viscosityand modify the behaviour of the polymer near the solid surface of thefillers. Moreover, the bonds between polymers and fillers improve themechanical properties but may also cause prejudicial premature curing(“structuring”) of the precursor compositions of the elastomers.

Non-reinforcing fillers interact extremely weakly with the siliconepolymer. These are, for example, chalk, quartz powder, diatomaceousearth, mica, kaolin, aluminas or iron oxides. Their effect is often toincrease the viscosity of the uncured precursors of the elastomers, aswell as the Shore hardness and the modulus of elasticity of theseprecursors.

Silicone elastomers may also contain, inter alia, catalysts, inhibitors,crosslinking agents, pigments, antiblocking agents, plasticizers andadhesion promoters.

These elastomers, curable by polyaddition and also called RTVelastomers, are formed, before curing, by casting, extrusion,calendering, or compression, injection or transfer moulding.

Silicone compositions made of elastomers, which can be cured bypolyaddition at room temperature or at higher temperatures (generally<200° C.), are conventionally packaged in the form of two-componentsystems, that is to say comprising two parts which are packagedseparately and have to be mixed at the time of use. In two-componentsystems, one of the components comprises the catalyst for thepolyaddition reaction. This catalyst is preferably of the platinum kind.It may, for example, be a platinum complex like the one prepared fromchloroplatinic acid and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane,according to U.S. Pat. No. 3,814,730 (Karstedt catalyst). Other platinumcomplexes are described in Patents U.S. Pat. Nos. 3,159,601, 3,159,662and 3,220,972. This component including the catalyst may also compriseonly one of the POS fluids of type I having Si-alkenyl, preferablySi-vinyl, crosslinking functional groups or only one of the POS fluidsof type II having an SiH crosslinking functional group. Generally, thePOS fluids of type I and the POS fluids of type II comprise at least twoSi-Vi and SiH groups per molecule, respectively, preferably in the α andω positions on the chain: at least one of the two having to comprise atleast three crosslinking functional groups per molecule. Thesecompositions comprise, in a known manner, POS fluids of type I and II, aplatinum catalyst for crosslinking by polyaddition and a platinuminhibitor allowing the compositions to cure only once they have beenremoved from the package and mixed together, optionally after they havebeen heated slightly. As examples of inhibitors, mention may be made of:

polyorganosiloxanes, advantageously cyclic polyorganosiloxanes,substituted with at least one alkenyl, tetramethylvinyltetrasiloxanebeing particularly preferred,

pyridine,

organic phosphines and phosphites,

unsaturated amides,

alkylated maleates

and acetylenic alcohols (cf. FR-B-1,528,464 and FR-A-2,372,874).

Such compositions may also be in the form of one-component systems whichcure only after having been heated.

The preparation of concentrated suspensions (pastes) of reinforcingsilicas in vinyl silicone oils, these suspensions being intended toproduce elastomers that can be cured by the reaction of apolyhydrogenated crosslinking molecule such as a POS with the vinylsilicone oil (SiH/SiVi addition), is widespread in the field ofelastomers.

The commonest reinforcing particulate fillers are based on silica, butsubstances such as TiO₂, Al₂O₃ and kaolin, for example, may also be usedin certain cases. These reinforcing fillers have a BET specific surfacearea of at least 50 m²/g, and generally up to 400 m²/g. These areultrafine powders which may be dispersed in silicone, preferably SiVi,oils. This dispersion causes problems when mixing some of thepulverulent filler with the oil and particular care must be taken inorder to obtain a uniform distribution of the fillers in the suspension.

Another difficulty to be overcome is associated with the rheology of thesuspensions prepared. This is because it is clear that introducing apulverulent particulate filler of very small particle size into thesilicone oil necessarily causes an appreciable increase in theviscosity. However, this characteristic, although it accompanies theachievement of good mechanical properties for the silicon elastomerscomprising the suspension as raw material, is prejudicial to thehandling and forming of the suspension and of the silicone compositionscontaining the suspension. It is in fact more convenient, for moulding,extrusion or forming, to handle fluid compositions which readily lendthemselves, inter alia, to pumping, flowing or mixing with functionaladditives.

The problematic considered here may therefore be summarized as how tofind a technical compromise between a priori antinomic specificationsfor the suspensions of fine particulate fillers in silicone oils,namely: fine distribution of particles in the silicone matrix—uniformityof the dispersion—suitability of the rheology of the suspension to thehandling constraints (processibility)—mechanical properties of the RTVsilicone elastomers.

French Patent Application No. 2,320,324 falls within this problematicand describes a process for a homogeneous distribution inpolyorganosiloxanes of a highly disperse active filler of BET specificsurface area of at least 50 m²/g, this process being characterized inthat the filler is treated during incorporation, in the presence ofwater, by a modifier or compatibilizer of the silazane type,hexamethyldisilazane being particularly preferred. The othercompatibilizers mentioned are trimethylsilane, trimethylchlorosilane,trimethylethoxysilane, triorganosilyl mercaptans, triorganosilylacylates or triorganosilyl amines. According to this process, describedin this prior application, an α,ω-trimethylsiloxy polydimethylsiloxanewith hexamethyldisilazane (HMDZ) and with water. Once this mixture hasbeen homogenized, some particulate silica is incorporated into it andmixing is continued until a homogeneous mixture is obtained. Next, themixture is heated to 130° C. in order to remove the excess HMDZ andwater by devolatilization. It is left to cool and, after measuring theviscosity of the suspension obtained, it is found that the latter isrelatively high, which, of course, gives the elastomers capable of beingprepared from this suspension good mechanical properties, but whichproves to be unacceptable from a handling standpoint in an industrialcontext. This compatibilization treatment of the silica with thesilicone oil may be termed “early” since the HMDZ is present as soon asthe reinforcing silica is brought into contact with this silicone oil.The compatibilization treatment is a means of retarding or preventingreaction between the surface of a reinforcing filler and a siloxanepolymer. This interaction causes what is called structuring and as aresult the conversion of these mixtures is more difficult.

Processes for preparing a suspension of reinforcing silica in siliconeoils are also known, in which the compatibilization treatment with theaid of hexamethyldisilazane is carried out after the silica has beenincorporated into the silicone oil. This method of treatment is termedhere “late”. It provides relatively fluid suspensions which may have atendency to structure over time. In certain cases, the suspensionsformed may have a certain tendency to be thixotropic. This is notwithout having undesirable consequences when converting and handlingthese suspensions, especially when degassing them. Nevertheless, it maybe stated that, whatever the rheological properties of the suspensionsobtained by late HMDZ treatment, the final mechanical properties of theelastomers prepared from the said suspensions are perfectible (hardness,tensile strength, elongation at break, tear strength). In particular,they do not reach the level of those found in the case of earlytreatment.

By way of illustration of this type of compatibilization treatment,mention may be made of European Patent Application No. 0,462,032 whichdescribes a process for preparing a paste which can be used especiallyin compositions that are curable by a polyaddition reaction and whichthus allows silicone elastomers to be obtained. According to thisprocess, the following are injected, continuously and simultaneously,into a twin-screw extruder, at at least four different points:

at least one SiVi POS oil,

water,

silica,

a liquid polysilazane under normal temperature and pressure conditions(HMDZ).

The silica is injected downstream of the water and of the oil andupstream of the HMDZ without, however, there being any mixing, betweenthe HMDZ and water on the one hand and between the HMDZ and the silicaon the other, before introduction of HMDZ.

U.S. Pat. No. 4,785,047 discloses a hybrid compatibilization treatment,at the boundary between the early and late treatments mentioned above.This patent relates more specifically to a process for preparingtransparent silicone elastomers. This document describes pumpable liquidcompositions formed from suspensions of HMDZ-treated siliceous filler insilicone oils which may or may not carry polyaddition-crosslinkingfunctional groups (SiH/SiVi). The problematic presented in this patentis different from that of the prior art presented above. In this case itis in fact more one of obtaining transparent elastomers and, in order todo so, of trying to counteract the deleterious effect of the siliceousfiller on the transparency by a very substantial HMDZ treatment.According to the process forming the subject of that patent, part of thesilicone oil is firstly mixed with all of the water and all of thesilica, but only with a fraction of the HMDZ representing systematicallymore than 15% by dry weight with respect to the silica, namely 34% and26% in the examples. After this first mixture has been homogenized, theremaining HMDZ is incorporated and mixed into the latter. Next, thedevolatilization treatment is carried out for 1 hour at 150° C. andunder reduced pressure. Finally, the rest of the PMDS silicone oil andthe α, ω-diVi PDMS silicone oil are mixed for 1 hour at roomtemperature. The transparent curable silicone suspension obtained has aviscosity lying between 200 and 10,000 Pa.s at 25° C. This technicalproposal may possibly provide a solution to the transparency problem,but it proves to be unsatisfactory with regard to the viscosity of thesuspension and to its handling.

BRIEF SUMMARY OF THE INVENTION

In such a technical context, one of the essential objectives of thepresent invention is to provide a process for preparing a suspension ofa particulate filler, treated with the aid of a compatibilizer, in asilicone oil, this suspension being able to be used as a raw materialfor the production of RTV elastomer compositions that can be cured bypoplyaddition.

This process has to meet the following specification:

the distribution of the filler in the silicone oil must be uniform andhomogeneous,

the dispersion must be optimal,

the suspension must flow well (no flow threshold) and the viscosity mustbe suitable for handling and converting the suspension,

the mechanical properties of the elastomers must be of an acceptablelevel.

Another essential objective of the invention is to provide a process forpreparing a reinforcing filler/silicone oil suspension for RTVelastomers which is simple to employ, inexpensive and able to be appliedon an industrial scale.

Another essential objective of the invention is to provide a process forobtaining a silicone composition, curable by polyaddition in order toform an RTV elastomer and comprising, as a constituent element, thesuspension as obtained by the intended process above.

These objectives, among others, are achieved by the present inventionwhich relates to a process for preparing a suspension of a particulate,preferably siliceous, filler in a material formed by a silicone oilcomprising:

polyorganosiloxanes (POS fluids) of type (I) which carrySi-alkenyl—preferably Si-vinyl—functional groups capable of reactingwith the Si—H crosslinking functional groups of a POS fluid of type II,

optionally, POS fluids of type (II) which carry Si—H crosslinkingfunctional groups capable of reacting with the Si-alkenyl functionalgroups of the POS fluids (I),

and/or, optionally, POS fluids of type (III) which differ from the POSfluids (I) and (II),

the said suspension being able to be used, in particular, for producingsilicone compositions that can be cured by polyaddition,

this process being of the kind of those in which the particulate filleris treated with the aid of a compatibilizing agent or compatibilizer(CA),

characterized in that it essentially consists in introducing somecompatibilizer (CA) into the preparation mixture:

on the one hand, before and/or substantially simultaneously with thecontacting of at least part of the silicone oil employed with at leastpart of the particulate filler used, this CA introduction taking placein one or more steps for a CA fraction corresponding to a proportion ofat most 8%, preferably at most 5% and even more preferably at most 3% bydry weight with respect to the total particulate filler;

and, on the other hand, after this POS/filler contacting.

It is to the credit of the inventors that they have demonstrated, afterextensive research and many experiments, that it is surprisingly andunexpectedly appropriate to incorporate the compatibilizer (for exampleHMDZ) before and after the reinforcing, preferably siliceous filler hasbeen mixed with the silicone oil, (preferably of SiVi type (I)), as longas the fraction of compatibilizer CA introduced before POS/filler mixingcorresponds to less than 5% by weight of the total reinforcing filler.

These novel and advantageous provisions make it possible to obtainsuspensions having suitable rheological properties and suitableviscoelastic behaviour. This is because these suspensions do not have aflow threshold, or have a very low threshold which is not prejudicial tothe applications. This considerably improves their processing. Inparticular, they have a fluidity which is stable over time and suitablefor the handling and conversion operations, such as pumping,transferring, mixing, forming, moulding, extrusion, etc.

One of the major advantages of the invention is that this attainmentfrom the rheology standpoint is not to the detriment of the finalmechanical properties of the crosslinked elastomer. The technicalcompromise is achieved.

Moreover, the methodology adopted makes it possible to obtain goodhomogeneous dispersions of the particulate filler in the oil. Inaddition, this methodology does not significantly complicate theprocess, which remains simple and inexpensive to implement.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with one of these preferred methods of implementation, theprocess according to the invention essentially consists:

in mixing:

100 parts by weight of silicone oil

0 to 5 parts by weight of water

20 to 80 parts by weight of particulate filler consisting of silica

1 to 20 parts by weight of compatibilizer (CA) selected from silazanestaken by themselves alone or as a mixture thereof, preferably fromdisilazanes, hexamethyldisilazane which may or may not be combined withdivinyltetra-methyldisilazane being particularly preferred;

in leaving the above to react, preferably with stirring,

in heating the mixture obtained, choosing a pressure/temperature pair sothat at least some of the water and of the volatile elements undergoesdevolatilization;

if necessary, in cooling the mixture.

In other words, the process according to the invention makes it possibleto control the viscosity of the suspension while at the same timemaintaining the mechanical properties of the final elastomer obtainedfrom the suspension at an acceptable level, or even improving thislevel.

The mixing is carried out with the aid of known and suitable devices.These may be, for example:

arm mixers

internal mixers

planetary mixers

ploughshare mixers

corotating or counterrotating twin-shaft mixers

continuous extruder-mixers

or other continuous or batch devices.

The mixing operation is carried out at normal temperature and pressureand preferably in an inert atmosphere (N₂). Moreover, under theseconditions the silicone oil, the water but also the compatibilizer arein liquid form in order to make the mixing easy.

The reinforcing, preferably siliceous, filler represents from 10 to 50%by weight of the suspension. In practice, this filler is of the order of30±10%.

Advantageously, the proportion of compatibilizer introduced in a firststep is at most equal to 8% of the reinforcing filler (and, for example,between 1 and 3% of the reinforcing filler, preferably between 1 and2%). Moreover, it may be pointed out that the total amount ofcompatibilizer CA is preferably between 5 and 30% of the siliceousfiller, preferably between 10 and 20%.

The proportions of compatibilizer AC introduced before and afterfiller/oil mixing are (5-25), preferably (10-20%), respectively.

In order to define more precisely the preferred method of implementingthe process according to the invention, without however this beinglimiting, it may be pointed out that the process comprises the followingsteps:

all or some of the silicone oil, the water and all or some of theparticulate siliceous filler are mixed with a first CA fraction ofbetween 1 and 3% by dry weight with respect to the silica,

a second CA fraction, representing between 10 and 15% by dry weight ofsilica, is incorporated into the mixture,

optionally, the rest of the silicone oil and the rest of the silica areadded,

the mixture is allowed to react, preferably by continuing the mixing,

the mixture is devolatilized, preferably in an inert-gas atmosphere (eg.N₂),

optionally, the devolatilized mixture is allowed to cool

and, optionally, the suspension is completed with the rest of thesilicone oil.

According to a first particular practical implementation of the processof the invention, it comprises the following steps:

1. a mixture comprising the silicone oil, the water and the firstCA—preferably HMDZ—fraction is homogenized,

2. the particulate filler, preferably silica, is gradually added to themixture obtained at 1,

3. the mixing is continued,

4. the second CA—preferably HMDZ—fraction is gradually incorporated intothe mixture obtained at 3,

5. the mixing is continued,

6. the mixture is devolatilized, preferably by heating to a temperature≧100° C.

In step 1 of this first practical implementation, a choice is madebetween, inter alia, the following three alternatives:

a) either all the oil and all the silica, as well as the initial CAfraction, are used,

b) or all the oil, part of the silica and the initial CA fraction areused,

c) or all the silica, part of the oil and the initial CA fraction areused.

According to a second particular practical implementation of theinvention, it comprises the following steps:

1′. the silicone oil and the water are homogenized,

2′. the particulate filler—preferably silica—and, at the same time, thefirst CA—preferably HMDZ—fraction are gradually incorporated into themixture obtained at 1,

3. the mixing is continued,

4. the second CA—preferably HMDZ—fraction is gradually incorporated intothe mixture obtained at 3,

5. the mixing is continued,

6. the mixture is devolatilized, preferably by heating to a temperature≧100° C.

The characteristic of this second method is associated with the factthat the process involves the co-addition of the particulate reinforcingfiller and its compatibilizer. It is therefore conceivable to make apreblend of these two constituents, or, alternatively, to introduce themconcomitantly. The gradual incorporation in step 2′may be carried outcontinuously or in stages.

According to a variant of this second practical method ofimplementation,

1″. the silicone oil is introduced,

2″. the particulate filler—preferably silica—together with the firstCA—preferably HMDZ—fraction and the water are gradually andsimultaneously incorporated into the oil,

3. the mixing is continued,

4. the second CA—preferably HMDZ—fraction is gradually incorporated intothe mixture obtained at 3,

5. the mixing is continued,

6. the mixture is devolatilized, preferably by heating to a temperature≧100° C.

According to one advantageous provision of the invention, correspondingto the case in which the reinforcing filler is silica and thecompatibilizer CA is HMDZ, a sufficient amount of HMDZ is used for thecontent of Si(Me)₃ units on the surface of the silica to be ≧1 Si(Me)₃unit per mm² and preferably between 1 and 2 Si(Me)₃ units per mm².

According to a third method of implementing the invention, the processto which it relates is characterized:

in that the first CA fraction is replaced, completely or partly, with atleast one processing aid chosen from molecules and combinations ofmolecules:

capable of interacting with the particulate filler, particularly withsilicon if a siliceous filler is used, to the detriment of the hydrogenbonds that this particulate filler establishes especially between itsown atoms and/or with those of the silicone oil,

and capable of being removed from the preparation mixture bydevolatilization,

and in that actions are taken to ensure that this processing aid is inthe presence of water in the preparation mixture.

In accordance with this third method of implementation, it is preferablefor the processing aid to be readily removable from the preparationmixture. For this purpose, it is beneficial for it to be easily removedby devolatilization, for example by heating in a vacuum or in a gasstream. Under these conditions, it clear that, as processing aid,molecules of low molecular weight will be preferred.

Advantageously, the processing aid is chosen from the group comprising:

silazanes, HMDZ being preferred;

difunctional, or preferably monofunctional, hydroxylated siloxanes;

amines, preferably ammonia and/or alkylamines, diethylamine beingparticularly preferred;

organic acids, formic and/or acetic acids being preferred;

and mixtures thereof.

As indicated above, the products more particularly selected asprocessing aids are those having a low molecular weight. This proves tobe the case especially for the amines and the organic acids mentionedabove.

With regard to the products employed in the process according to theinvention, it may be pointed out that, in the case of the silicone oil,linear or cyclic, but more especially linear, polydiorganosiloxanes willpreferably be chosen.

With regard to the POS fluids (I), these will be polydiorganosiloxaneoils carrying an Si-alkenyl, particularly an Si-vinyl, group in and/orat the ends of the chain. In practice, mention may be made, for example,of α,ω-divinyl-terminated polydialkyl (methyl) siloxanes. Preferably,the POS (I) used for preparing the suspension is a vinyl POS (I)carrying at least two SiVi units per molecule, preferably at least threeper molecule, when the POS (II) contains only two SiH units permolecule.

As regards the POS (II), this is chosen frompolyorganohydrogenosiloxanes comprising at least two SiH units permolecule, preferably at least three, when the POS (I) comprises only twoSiVi units per molecule. In practice, mention may be made, for example,of polyalkyl(methyl)hydrogenosiloxanes or else branched hydrogenated POSfluids having trifunctional or tetrafunctional units and units carryingSiH.

The POS (III) may be a polydiorganosiloxane such as a polyalkylsiloxane,preferably a polydimethylsiloxane, having trimethylsilyl end groups.

The preferred silicone oils (I, II, III) essentially comprise R₂SiOunits, the symbols R, which may be identical or different, representingC_(1-C) ₄ (cyclo)alkyls which may or may not be halogenated, or arylgroups, which may or may not be substituted or halogenated.

By way of groups:

alkyl: mention may especially be made of methyl, ethyl, propyl and butylgroups,

haloalkyl: mention may be made of 3,3-trifluoropropyl,

cycloalkyl: mention may be made of cyclohexyl,

aryl: mention may be made of the phenyl group.

Preferably, at least 85% of the groups R represent methyl groups.

The silica used in the process according to the present invention is areinforcing silica whose specific surface area is preferably between 50and 400 m²/g. These silicas may be precipitated silicas, but moregenerally fumed silicas are employed. The fact that silica is preferreddoes not exclude making use of other types of known reinforcing filler.

The CA is preferably a silazane and even more preferably a disilazane.This is a product which is liquid under standard temperature andpressure conditions (23° C./760 mmHg).

The viscosity of the suspension is one of the key parameters whichgovern the process according to the invention. Thus, in accordance withone advantageous provision of the latter:

alkenylated—preferably vinylated—silicone oil comprising at least twoSi-alkenyl groups per molecule, each preferably located at one end ofthe chain, and having a dynamic viscosity at 25° C. not exceeding 250Pa.s, preferably not exceeding 100 Pa.s and more preferably still notexceeding 10 Pa.s, is employed,

a silica having a BET specific surface area of between 50 and 400 m²/gand mixing conditions such that the dynamic viscosity at 25° C. of thesuspension does not exceed 300 Pa.s, preferably does not exceed 250 Pa.sand more preferably still does not exceed 200 Pa.s, are chosen.

INDUSTRIAL APPLICATION

The purpose of the reinforcing filler/silicone oil suspension preparedin accordance with the invention is for it to be used for obtainingliquid or pasty silicone compositions made of RTV silicone elastomer,which compositions can be cured, preferably by polyaddition, in theambient atmosphere and at a normal temperature or at a highertemperature.

Thus, according t o another of these aspects, the present inventionrelates to a process for obtaining a silicone composition that can becured by polyaddition, characterized in that it consists in mixing thefollowing products:

A—a suspension as prepared according to the process as defined above,

B—one or more POS fluids (I), as defined above,

C—one or more POS fluids (II), as defined above,

D—optionally, one or more POS fluids (III), as defined above, useful asdiluent (s)

E—a catalytic system comprising a catalyst, preferably of the platinumkind, and, optionally, an inhibitor or retarder.

According to a first variant of this process:

the composition is produced in the form of a two-component systemcomprising parts C₁ and C₂ which are intended to be brought into contactwith each other in order to produce an elastomer crosslinked bypolyaddition between the POS fluids (I) and (II),

and care is taken to ensure that only one of the parts, C₁ or C₂,contains some catalyst D and, optionally, one or other of the POS fluids(I) and (II)

According to a second variant of this process for preparing curableliquid compositions, a one-component system is produced which isintended to be crosslinked in the ambient air and/or under the effect oftemperature.

These curable compositions, which are precursors of elastomers, may alsocomprise one or more functional additives F such as, for example, anon-reinforcing filler formed by chalk, quartz powder, diatomaceousearth, mica, kaolin, aluminas or iron oxides. These optional additives Fmay also consist of pigments, antiblocking agents, plasticizers orrheology modifiers, stabilizers or adhesion promoters.

The examples which follow illustrate:

the preparation of suspensions of reinforcing filler in silicone oils inaccordance with the invention,

the application of these suspensions as raw material for obtainingtwo-component curable compositions made of RTV silicone elastomers,

and the evaluation of the viscoelastic properties of the suspensions andthe mechanical properties of the crosslinked elastomers obtained fromthe said suspensions.

Two methods of implementing the process of the invention are given inthe examples. The latter also comprise comparative examples forpreparing suspensions accordance with the prior art usingcompatibilization treatment methods of the “late” type and of the“early” type.

EXAMPLES Comparative Example 1 Late Treatment

Introduced into a 1.5 1 arm mixer are 750 g of α, ω-divinyl-terminatedPolyDiMethylSiloxane (PDMS) oil having a viscosity of 0.6 Pa.s and 21 gof water. After homogenization, 321 g of a fumed silica, characterizedby its specific surface area of 300 m²/g, are added in portions over 70minutes. After mixing for 120 minutes, 66 g of hexamethyldisilazane areadded over 90 minutes. A heating phase starts 60 minutes later, duringwhich, when the temperature reaches 80° C., the mixture is placed in astream of nitrogen (250 l/h); the heating continues until reachingapproximately 155°, a steady temperature which is maintained for 2 h.After cooling, 43 g of the vinyl-terminated oil are cooled and thesuspension homogenized.

Starting from this suspension, a part A and a part B are formulated.

Part A contains:

90.6 g of the suspension;

1.58 g of the α,ω-divinyl-terminated PDMS oil described;

5.48 g of an α,ω-dihydrogeno PDMS oil containing 1.9 meq SiH per gram ofoil

2.35 g of a polyhydrogeno PDMS oil having a viscosity of 30 mPa.s,containing 1.6 meq SiH per gram of oil.

Part B contains:

11 g of the suspension;

29 g of the α,ω-divinyl-terminated PDMS oil described above;

9.77 g of an α,ω-divinyl-terminated PDMS oil containing 0.05 meq Vi pergram of oil;

70 μl of a Karstedt catalyst containing 12% platinum;

90 μl of divinyltetramethyldisiloxane;

140 μl of tetravinyltetramethylcyclotetrasiloxane.

Parts A and B are mixed in a ratio of 100 to 10 and, after degassing, 2mm thick plaques of elastomers are prepared. The mouldings are cured ina ventilated oven for 1 hour at 150° C. The test pieces necessary formeasuring the mechanical properties are cut from these plaques of curedelastomer.

Example 2 Early Treatment

The previous example is repeated except that the process starts with themixer being charged with 750 g of α,ω-divinyl-terminated PDMS oil, 21 gof water and 66 g of hexamethylsisilazane [sic]. After stirring for 10minutes, the silica is incorporated in portions over 30 minutes; themixing is continued for a further 120 minutes before starting theheating phase, which is the same as above. The suspension is formulatedas in Example 1.

Comparative Properties of the Suspensions and of the Cured ElastomersAccording to Examples 1 and 2

The viscosity of the suspensions is measured by means of a dynamicrheometer with a cone/plate geometry. The complex viscosity at 1 Hz andat 1 Pa is taken as being representative of the viscoelastic behaviourof the suspensions. The mechanical properties are measured according tothe standards in force:

DIN 53505 for the hardness measurement

AFNOR T46002 for the breaking measurements.

Dynamic Shore A Stress Elongation viscosity hardness at break at breakPa.s pts MPa % Example 1  13 38 6.1 390 Example 2 3000 35 6.9 560

In the case of Example 1, the viscosity is low but the breakingproperties of the elastomer are moderate. With regard to Example 2, thishas better breaking properties, but at the price of having a highviscosity.

Example 3 Two-Step Treatment

Introduced into a 100 1 arm mixer are 40 kg of α,ω-divinyl-terminatedoil having a viscosity of 2 Pa.s, 0.27 kg of hexamethyldisilazane and0.27 kg of water. After homogenization, 16.2 kg of a fumed silicacharacterized by its specific surface area of 200 m²/g are added inportions over 100 minutes. After 60 minutes of mixing, 1.9 kg ofhexamethyldisilazane are added over 60 minutes. A heating phase isstarted 120 minutes later, during which the mixture is placed in astream of nitrogen (30 m³/h); the heating continues until reachingapproximately 140°, a steady temperature which is maintained for 2 h.The suspension is then left to cool.

Starting from this suspension, a part A and a part B are formulated.

Part A contains:

427 g of the suspension;

10 g of a polyvinyl PDMS oil having a viscosity of 0.4 Pa.s, containing0.11 meq Vi per gram of oil;

25 g of trimethyl-terminated oil having a viscosity of 0.1 Pa.s;

26.5 g of an α,ω-dihydrogeno PDMS oil, containing 1.9 meq SiH per gramof oil;

11.3 g of a polyhydrogeno PDMS oil having a viscosity of 30 mPa.s,containing 1.6 meq SiH per gram of oil.

Part B contains:

180 g of the suspension;

20 g of the methyl-terminated oil described;

250 μl of a Karstedt catalyst containing 12% platinum;

1 ml of tetravinyltetramethylcyclotetrasiloxane.

Parts A and B are mixed in a ratio of 100 to 10 and, after degassing,the test pieces necessary for measuring the mechanical properties areprepared as explained in Comparative Examples 1 and 2.

Example 4 Two-Step Treatment with Coaddition of the Reactants

The previous example is repeated except that the mixer is firstlycharged with 40 kg of α,ω-divinyl-terminated PDMS oil and 0.27 kg ofwater. After stirring for 10 minutes, the silica is incorporated inportions over 120 minutes at the same time as 0.27 kg ofhexamethyldisilazane, which is divided according to the portions ofsilica. After this phase of silica and hexamethyldisilazane coaddition,the process is continued as previously. The suspension is formulated asin Example 3.

Properties of the Suspensions and of the Cured Elastomers According toExamples 3 and 4

The viscosity of the suspensions is measured by means of a dynamicrheometer with a cone/plate geometry. The complex viscosity at 1 Hz andat 1 Pa and the threshold stress, for which the elastic and viscousmoduli are equal, are taken as being representative of the viscoelasticbehaviour of the suspensions. The mechanical properties are measuredaccording to the standards in force.

Dynamic Threshold Shore A Tear viscosity stress hardness strength Pa.sPa pts N/mm Example 3 165 25 34 21 Example 4 150  1 36 18

Examples 3 and 4 show that the elastomers prepared from the oil/silicasuspensions obtained in accordance with the invention have a rheologywith a very low flow threshold and a moderate viscosity. This veryfavourable Theological behaviour is accompanied by excellent tearstrength.

Example 5 Two-Step Treatment by Hexamethyldisilazane

Preparation of the Suspension

Introduced into a 7 l arm mixer are 2120 g of a mixture ofα,ω-divinvl-terminated oils having a viscosity of 1.5 Pa.s, 12.6 g ofwater and 12.6 g of hexamethyldisilazane. After homogenization, 765 g ofa fumed silica characterized by its specific surface area of 200 m²/gare added in portions over 110 minutes. Then, 80 g ofhexamethyldisilazane are added over 60 minutes. A heating phase starts120 minutes later, during which, when the temperature reaches 70° C.,the mixture is placed under vacuum; the heating continues until reachingapproximately 150° C., a steady temperature which is maintained for 1 h.The mixture is then cooled in a stream of nitrogen (≈250 l/h) and theapparatus is drained.

Starting from this suspension, a part A and a part B are formulated.

Part A contains;

946.7 g of the suspension;

42.7 g of an 60,ω-dihydrogeno oil, containing 1.9 meq SiH per gram ofoil;

10.7 g of a polyhydrogeno oil, containing 1.6 meq SiH per gram of oil.

Part B contains:

22 g of the suspension;

20 g of an α,ω-divinyl-terminated oil, containing 0.15 meq Vi per gramof oil;

58 g of an α,ω-divinyl-terminated oil, containing 0.05 meq Vi per gramof oil;

140 μl of a Karstedt catalyst containing 10% platinum;

0.15 g of divinyltetramethyldisiloxane;

0.35 g of tetravinyltetramethylcyclotetrasiloxane.

Parts A and B are mixed in a ratio of 100 to 10 and, after degassing,the test pieces necessary for measuring the mechanical properties areprepared.

Example 6 Treatment with an Acid First Step

The previous example is repeated except that the 12.6 g corresponding tothe first hexamethyldisilazane portion are replaced with 3.6 g of formicacid. All the other operations are carried out as previously. Thesuspension is formulated as in Example 5.

Example 7 Treatment with a Base First Step

Example 5 is again repeated, except that the 12.6 g corresponding to thefirst hexamethyldisilazane portion are replaced with 4.2 g of aqueousammonia containing 32% ammonia. All the other operations are carried outas previously. The suspension is formulated as in Example 5.

Example 8 Evaluation of the Comparative Properties of the Suspensions ofExamples 5 to 7

The viscosity of the suspensions is measured by means of a dynamicrheometer with a cone/plate geometry. The following are taken as beingrepresentative of the viscoelastic behaviour of the suspensions:

the complex viscosity and its elastic and viscous components at 1 Hz andat 1 Pa;

the threshold stress for which the elastic and viscous moduli are equal.The mechanical properties are measured according to the standards inForce.

Acid Base Example 5 Example 6 Example 7 Complex viscosity 41 38 42Elasticity factor 0.68 0.65 0.73 Flow threshold <1 <1 <1 Shore Ahardness 28 35 32 Elongation at break 700 400 610 Tensile strength 6.86.3 8.0 Tear strength 29 21 23

It may readily be seen that the Theological behaviour of the suspensionsis, in the three cases, typical of a product which flows well and thatthese suspensions allow elastomers to be produced with good mechanicalproperties.

What is claimed is:
 1. A process for preparing a suspension of aparticulate filler in a material, wherein said material is prepared froma silicone oil comprising: polyorganosiloxanes (POS fluids) of type (I)which carry Si-alkenyl functional groups capable of reacting with theSi—H crosslinking functional groups of a POS fluid of type II,optionally, POS fluids of type (II) which carry Si—H crosslinkingfunctional groups capable of reacting with the Si-alkenyl functionalgroups of the POS fluids (I), and/or, optionally, POS fluids of type(III) which differ from the POS fluids (I) and (II), said suspensionbeing able to be used for producing silicone compositions that can becured by polyaddition, and wherein said particulate filler is treatedwith a compatibilizer; said process comprising, forming a preparationmixture of the particulate filler and the silicone oil, and introducingcompatibilizer into the preparation mixture either: before and/orsimultaneously with the contacting of at least part of the silicone oilemployed with at least part of the particulate filler used, thisintroduction of compatibilizer taking place in one or more steps or,after the contacting of at least part of the silicone oil employed withat least part of the particulate filler used; and wherein: all or someof the silicone oil, water and all or some of the particulate filler aremixed with a first compatibilizer fraction of between 1 and 3% by dryweight with respect to the filler, a second compatibilizer fraction,representing between 10 and 15% by dry weight of filler, is incorporatedinto the mixture, optionally, the rest of the silicone oil and the restof the are added, the mixture is allowed to react, the mixture isdevolatilized, optionally, the devolatilized mixture is allowed to cool,and optionally, the suspension is completed with the rest of thesilicone oil.
 2. The process according to claim 1, wherein: a mixturecomprising the silicone oil, the water and the first compatibilizerfraction is homogenized, the particulate filler is gradually added tothe mixture obtained, mixing is continued, the second compatibilizerfraction is gradually incorporated into the mixture obtained from theprevious step, mixing is continued, and the mixture is devolatilized. 3.The process according to claim 1, wherein; the silicone oil and thewater are homogenized, the particulate filler and, at the same time, thefirst compatibilizer fraction are gradually incorporated into themixture obtained from the previous step, mixing is continued, the secondcompatibilizer fraction is gradually incorporated into the mixtureobtained from the previous step, mixing is continued, and the mixture isdevolatilized.
 4. The process according to claim 1, wherein: thesilicone oil is introduced, the particulate filler together with thefirst compatibilizer fraction and the water are gradually andsimultaneously incorporated into the silicone oil, mixing is continued,the second compatibilizer fraction is gradually incorporated into themixture obtained from the previous step, mixing is continued, and themixture is devolatilized.
 5. The process according to claim 1, whereinsaid particulate filler is silica.